AppyDays is a free app that helps you to find discounted (and free) apps. Search filters for device type (Mac, iPhone, and iPad are supported), app rating, category, free-ness, and popularity are available, in addition to a blacklist system that allows you to ignore discounts you have already taken advantage of or are not interested in. You can blacklist on a per-app basis, and on a per-developer basis as well, if there's a recurring company whose cheap junk isn't even worth the download. Results can be refreshed at will, although even refreshed results do not always keep pace with App Store prices - more than once I have clicked through on a link only to find that the discount no longer applies. Yet this is rare, and the service AppyDays provides is on the whole an invaluable one for the digitally inclined cheapskates among us.
There are a couple other features and flaws worth mentioning. It is possible to add an app to a "Favorites" list, which you can use to track the prices of specific apps; however, it must be on the discount list for your selected platform to be added to the favorites in the first place. The app rating isn't always available when it should be, and it's unclear how the app's "Popularity" category is determined. But it's possible to edit the blacklist directly which is handy, and prices can be displayed in USD or in euros.